1. Field of the Invention
This invention relates to image encryption, and in particular, it relates to a method and apparatus for encrypting an image using an encryption key and embedding the encrypted image in a cover image. It also relates to methods and apparatuses of applying image encryption in scanning, printing, and electronic transmission of images.
2. Description of the Related Art
Scrambling of image signals has been used in video transmission and distribution as well as other areas to prevent unauthorized use. Scrambling generally refers to shifting the locations of lines and/or blocks on an image (e.g. video screen) so that the image is not easily recognizable. For example, in U.S. Pat. No. 6,535,687, “Method and system for deterring electronic video piracy through image rearrangement”, blocks of 3×3 pixels are rearranged using a pseudo random number sequence of numbers 1 to 9. The encryption method is implemented in a projector. U.S. Pat. No. 5,815,572, “Video scrambling”, teaches scrambling video data using a combination of line reversal, line permutation, line inversion, and block permutation. The permutation tables and other information needed for unscrambling are stored in the receiver. In U.S. Pat. No. 5,583,941, “Imaging processor”, pixel blocks are divided into 2×2 sub-blocks which are transposed diagonally to achieve scrambling. Different levels of scrambling are performed using different block sizes. The scrambling process is implemented in a printer. In U.S. Pat. No. 5,535,275, “Apparatus and method for producing scrambled digital video signals”, the screen is divided into 5×10 “super-blocks” each made up of 27 macroblocks, each macroblock consisting of 6 DCT blocks Y1 Y2 CR Y3 Y4 CB. A DCT block is an 8×8 pixel area. The macroblocks are read out in a shuffled sequence, where super-blocks form five columns labeled “A B F D E” and are read out one row at a time in the column sequence of “F B D A E” (see FIG. 13 and cols. 8-9). This patent also teaches further scrambling by exchanging the data order of the Y, CR and CB components (see FIGS. 1-4 and cols. 9-10), as well as bit manipulation of the DCT coefficients (see FIGS. 5-7 and cols. 10-11). U.S. Pat. No. 5,321,748, “Method and apparatus for television signal scrambling using block shuffling”, describes video scrambling in which video lines are shuffled within a block of lines. One pseudo random number sequence (e.g. a sequence of numbers 1 to 8) is used for each block of lines and the seed numbers for the pseudo random number sequences are inserted into the video date stream. U.S. Pat. No. 5,208,85, “Method and device for scrambling-unscrambling digital image data”, describes scrambling of digital image where each block of N×N pixels is divided into 4 sub blocks and inter-block and intra-block scrambling are performed. Inter-block scrambling exchanges two sub blocks that are located diagonally. Intra-block scrambling exchange pairs of pixels that are located at mirror image locations with respect to a line.
Other scrambling techniques applies scrambling after the pixels have been transformed (e.g. by discrete cosine transform), such as the method described in U.S. Pat. No. 6,505,299, “Digital image scrambling for image coding systems”, and the method described in “Efficient Frequency Domain Selective Scrambling of Digital Video”, IEEE Trans. Multimedia, Vol. 5, No. 1, 118-128, March 2003.
Embedding data in a carrier data is known. For example, U.S. Pat. No. 6,023,511, “Cryptosystem for encrypting digital image or voice file”, describes embedding data representing encrypted image in the low bits of a camouflage image.